Along with demands for making hybrid automobiles and electric automobiles more compact, there is also a demand for reducing the size of components used in such automobiles, such as power conversion devices. Now, by mounting such a power conversion device to a vehicle component such as a transmission or a motor or the like, it is possible to reduce the system cost including the cost of the power conversion device, and to make the system more compact. However, if such a vehicle component to which a power conversion device is mounted is a component that generates heat, then it is necessary to solve thermal problems of components mounted to the power conversion device, such as a capacitor module for smoothing, since the power conversion device receives heat generated by the vehicle component and accordingly the temperature of the cooling water for the power conversion device and its surrounding temperature become high. In this connection, in Patent Document #1, a flow conduit defining member in which a coolant flow conduit for conducting coolant is defined is provided within a metallic housing, so that components that are mounted to the power conversion device are protected from heat transmitted from a vehicle component such as a transmission or a motor-generator.
However, under the supposition that the power conversion device is mounted to some vehicle component, there is a requirement for making more compact and for reducing the space further by employing a construction in which the device is matched to the shape of the vehicle component, which may for example be circular.